The control unit manages all actions in the CPU, taking inputs and directing the processor. It has sub-components that used to be wired but now use microcode. Its responsibilities vary depending on CPU architecture.
A control unit is the sub-component of the central processing unit (CPU) that manages all actions performed in this area in a computer. It is responsible for taking the various inputs from the computer, instructions and data and telling the processor what to do with them. Since the CPU is considered the brain of the computer, it is sometimes referred to as the brain within the brain. Depending on the architecture of the CPU, the control unit can have different tasks to perform.
The control unit is actually made up of several sub-components. During the wired days, all of this wiring and circuitry formed what is known as a finite state machine, a system that serves a singular purpose in directing computer operations. Separate circuits were responsible for decoding and encoding instructions, while others handled the logic or counting instructions that the CPU worked on. It all happened in order, where the logic circuit would be flipped one way or another to route instructions to storage.
An instruction is fetched and decoded, then has to be executed in order, one after the other until completion. In older CPUs, the instruction would have to run the whole process and finish the computation before the next one started. To speed up processing, modern CPUs use so-called pipelines, where each step is part of the pipeline. While one instruction is in the executing portion of the pipeline, another is already decoding and another is fetching. To handle all of this, the controller also had to act as a multiplexer, taking multiple inputs or outputs and routing them in and out of the pipeline.
As computer CPUs continued to advance, much of this changed dramatically. The use of microcode, small programs that reside in special high-speed read-only memory on the CPU, has taken the place of the old hardwired circuitry. These low-level programs took on the time-consuming work of physically rewiring a control unit and simplified CPU architecture changes. The control unit’s custom microprograms, created during the CPU design phase, are what enable the architecture of a particular type of CPU.
In general, much of the control unit’s responsibilities depend on the CPU architecture. Some can simply fetch, decode, coordinate execution, and direct the output of instructions. Others may have additional responsibilities involving translation, which can slow down the CPU. In these cases, the control unit can be further divided into succinct components, such as a separate scheduling unit, or a retirement unit that organizes and stores the results from the rhythmic logic unit (ALU).
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